How do I copy/paste pose in blender 2.83
Select Bone-6-8 (upper arm) of Armature.001, go in the Dope Sheet, select all the keyframes, copy, then select Armature.003, and paste in the Dope Sheet, the animation is copied. You could also simply switch the Dope Sheet to Action Editor mode and select the animation if you want to use the exact same animation.